1. Field of the Invention
This invention relates to a method for producing ultra-high purity, optical quality glass articles. More particularly, as described in full detail below, the invention involves: (1) using a sol-gel process to form fused silica granules, (2) preparing a green body from the granules, (3) purifying and consolidating the green body, and (4) subjecting the consolidated green body to hot isostatic pressing ("hipping") to produce the desired finished product.
2. Description of the Prior Art
Numerous investigators have attempted to apply the sol-gel technique to the production of optical quality glass products.
For example, Matsuyama et al., UK patent application No. GB 2,041,913, describes a gel casting method for producing "mother rods" from which optical waveguide fibers can be prepared wherein a solution of a silicon alkoxide is formed, allowed to gel so as to produce a porous preform, dried, and then sintered at a temperature below its melting temperature to produce the mother rod. The application describes a three step sintering process in which an atmosphere of oxygen and helium is used up to a temperature of 700.degree. C., an atmosphere of chlorine and helium is used between 700.degree. C. and 1000.degree. C., and an atmosphere of just helium is used above 1000.degree. C. As acknowledged in this application, drying the gel without cracking is difficult and can take as long as 10 days.
U.S. Pat. No. 4,419,115 to David W. Johnson, Jr., et al., describes a similar process for producing glass articles wherein fumed silica is mixed with a polar liquid to form a first sol, the first sol is gelled to form a first gel, the first gel is dried, heated to a temperature in the vicinity of 750.degree.-850.degree. C., cooled, redispersed in a polar liquid to form a second sol, the second sol is gelled to form a second gel, the second gel is dried, and the dried second gel is sintered to form the glass article.
The Johnson et al. patent states that the heating of the first gel to 750.degree.-850.degree. C. does not result in densification of the gel material. Specifically, the patent states that until final sintering, the BET surface area of its silica material remains essentially the same as that of fumed silica. With regard to sintering, the patent states that a helium atmosphere, which optionally contains chlorine, or a vacuum can be used during this step. Significantly, the patent employs the helium plus chlorine approach, and not the vacuum approach, in each of its examples. In practice, the process of the Johnson et al. patent, like the process of the Matsuyama et al. application, has been found to be subject to gel cracking problems.
In addition to the foregoing, sol-gel casting processes have also been described in Hansen et al., U.S. Pat. No. 3,535,890, Shoup, U.S. Pat. No. 3,678,144, Blaszyk et al., U.S. Pat. No. 4,112,032, Bihuniak et al., U.S. Pat. Nos. 4,042,361, and 4,200,445, and Scherer, U.S. Pat. No. 4,574,063, European patent publication No. 84,438, and Scherer et al., "Glasses from Colloids", Journal of Non-Crystalline Solids, 63: 163-172 (1984).
In particular, the Hansen et al. patent relates to a process in which an aqueous solution of colloidal silica particles is formed, dried to produce a gel, and the gel is sintered in a three step process, the first step comprising heating the gel to around 600.degree. C. in a vacuum, the second step comprising flushing the gel with chlorine gas to remove bound water, and the third step comprising sintering the gel under vacuum by raising its temperature to 1200.degree. C. The patent acknowledges the gel's high sensitivity to cracking during the drying process and states that drying times on the order of many days or weeks are needed to overcome this problem.
The Shoup patent, as well as the Blaszyk et al. patent, relate to a process in which gels are formed from soluble silicates, such as, alkali silicates. The dried gels can be used, for example, as filters, solid supports for catalysts, and the like, or can be consolidated into a solid glass body at temperatures ranging from 600.degree.-1700.degree. C. The gels produced by the soluble silicate technique are generally stronger than those produced by other sol-gel procedures. This makes crack-free drying of the gel easier and also facilitates the production of large castings. Alkali silicate solutions, however, contain significant amounts of iron. Accordingly, a leaching step is required if high purity glass is to be produced. Leaching is also generally required if the final product is to be alkali-free. In one set of examples, the Shoup patent compares consolidating a gel in air with consolidating a gel under a reduced pressure of 20 mm. In some cases, the reduced pressure resulted in a consolidated product which did not include bubbles; in other cases, bubbles still remained.
The Bihuniak et al. patents describe processes for densifying fumed silica and other fumed metal oxides by forming a sol, drying the sol to form fragments, and densifying the fragments by calcining them at 1150.degree.-1500.degree. C. Thereafter, the densified material can be milled, e.g., to an 8 to 10 micron average particle size, suspended in a casting medium, slip cast to form a porous preform, and fired to produce the desired finished product.
Because it employs fumed silica, the Bihuniak et al. process is more difficult to perform than the process of the present invention. For example, it is relatively difficult to form gels from fumed silica, and as acknowledged in the Bihuniak et al. patents, once formed, gels made from fumed silica tend to break up into large chunks, rather than small particles, as is desired. Further, extensive pollution abatement equipment is required to produce fumed silica since such production involves the creation of hydrochloric acid.
In addition, densified silica particles made from fumed silica tend to have higher impurity levels than the densified silica particles produced by the process of the present invention. These higher impurity levels are due in part to the fact that impurities, including trace amounts of radioactive materials, are introduced into the silica during the fuming process.
The higher impurity levels also arise from the fact that densification of particles made from fumed silica gels requires higher temperatures than densification of particles formed from gels prepared in accordance with the present invention, i.e., densification of particles made from fumed silica gels require temperatures above, rather than below, 1150.degree. C. Such higher temperatures generally mean that metal-containing furnaces must be used to perform the densification. The use of such furnaces, in turn, means that the silica particles will be exposed to and thus will pick up metal ions which are released from the walls of the hot furnace. In addition to the purity problem, the need to generate higher temperatures to achieve densification is in general undesirable.
The Scherer references describe forming a gel from fumed oxides in a non-aqueous medium, e.g., an organic medium, drying the gel, exposing the dried gel to vacuum for a few hours and heating the gel in oxygen to remove residual organic constituents, and then sintering the gel in a helium or helium plus chlorine atmosphere.
As with various of the sol-gel techniques described above, the gels produced by the Scherer technique are relatively fragile and thus must be carefully handled to avoid cracking. Also, as is typical of processes in which gels are sintered, gels prepared in accordance with the Scherer process undergo a linear shrinkage of approximately 40% upon sintering. Such a shrinkage level makes it relatively difficult to cast complex shapes and also leads to relatively high levels of gel fracture during sintering. In addition to the foregoing, because the Scherer process uses fumed silica, it suffers from the impurity and pollution control problems associated with the fuming process (see discussion above).
The use of hot isostatic pressing ("hipping"), as well as other pressing techniques, to compress gas bubbles in vitreous materials has been described in a number of references. See Rhodes, U.S. Pat. No. 3,310,392, Bush, U.S. Pat. No. 3,562,371, Okamoto et al., U.S. Pat. No. 4,358,306, and Bruning et al., U.S. Pat. No. 4,414,014 and UK patent application No. 2,086,369. The Bush patent, in particular, discloses forming a green body, sintering the body in a vacuum, and then subjecting the consolidated body to isostatic pressure at a temperature equal to or greater than the sintering temperature.